Pierre, the proposition, "I am able to simulate 3^^^^3 people" is not mutually exclusive with the proposition "I am able to simulate 3^^^^3-1 people."

If you meant to use the propositions D_N: "N is the maximum number of people that I can simulate", then yes, all the D_N's would be mutually exclusive. Then if you assume that P(D_N) ≤ P(D_N-1) for all N, you can indeed derive that P(D_3^^^^3) ≤ 1/3^^^^3. But P("I am able to simulate 3^^^^3 people") = P(D_3^^^^3) + P(D_3^^^^3+1) + P(D^^^^3+2) + ..., which you don't have an upper bound for.